1. Field of the Invention
The present invention relates to a non-aqueous electrolyte secondary battery, an electrode used for the secondary battery, and a method of manufacturing the electrode.
2. Description of the Related Art
In recent years, a non-aqueous electrolyte secondary battery has attracted attention as a power source of a hybrid electric vehicle or electric storage device for an electric generator using green energy such as solar energy and wind power. In such uses, the temporal variation in the load or generated power is dramatic, and hence a secondary battery with high capability for instantly storing or releasing a large current, i.e., a secondary battery with a exceptional high-current characteristics is demanded.
In general, a non-aqueous electrolyte secondary battery is provided with an exterior member or a container, formed of a metal, a laminate film or the like, an electrode body contained in the container together with an electrolyte liquid, and electrode terminals connected to the electrode body and exposed to the outside.
The electrode body is configured by superimposing a negative electrode plate obtained by forming a negative electrode active material layer on a current collector plate and a positive electrode plate obtained by forming a positive electrode active material layer on a current collecting plate upon each other with a separator interposed between the electrodes, and rolling up or stacking the resultant. Further, tabs for current collection extend from a side edge of the current collector plate (for example, Jpn. Pat. Appln. KOKAI. Publication No. 2006-139919).
In such a secondary battery with a high degree of high-current characteristics as described above, in order to make the reduction of the voltage as small as possible when a large current is made to flow, it is necessary to make the resistance of the active material layer small. However, at this time, the following problems are caused.
One of them is a problem due to the reduction in the resistance of the active material layer. It is the fact that in the electrode body constituted of a positive electrode, negative electrode, and separator for electrically separating these electrodes from each other, when the positive electrode and negative electrode are short-circuited by a minute conductive foreign substance through the separator for some reason, a minute leakage current flowing there becomes relatively larger by an amount corresponding to the reduction in the resistance of the active material layer. As a result of this, current leakage occurs even when the battery is not used, and the battery capacity is rapidly decreased from the normal level.
The second is a problem of electrode strain occurring in a coil-shaped electrode body formed by rolling up the electrodes into a cylinder or flat shape, when the electrode body is subjected to an electrode rolling/pressing process to reduce the resistance thereof. The electrode body formed by rolling up the electrodes into a cylindrical or flat shape is provided with a large number of current collection tabs. By increasing the number of tabs, it is possible to increase the cross-sectional area of the tabs through which the current flows and, consequently, the resistance decreases. However, in the electrode rolling/pressing process, pressure is applied to the active material layer, whereas no pressure is applied to the tabs since the roll of the rolling press is not brought into contact with the tabs. As a result of this, strain is caused at the boundary between the active material layer to which pressure is applied, and the tabs to which no pressure is applied. When such a strained electrode is rolled up, a gap is formed between the electrodes of the rolled-up coil, and the resistance component is increased.
As methods of forming an electrode with current collection tabs, there are provided a method of welding tabs to that parts of a current collector constituted of metallic foil, which are coated with the active material layer, and a method of punching uncoated parts of a current collector constituted of metallic foil to thereby forming tabs integral with the current collector. The number of tabs can be easily increased in the latter method, and hence the latter method is advantageous to a lithium ion secondary battery for the large current use.
The latter method is disclosed in, for example, Jpn. Pat. Appln. KOKAI Publication No. 2006-139919. According to this method of manufacturing a battery electrode, a width of an uncoated part (part on which no active material layer is arranged) continuously formed at a side edge part on the surface of a current collector in the longitudinal direction of the electrode is made 1 to 10 mm, a large number of tabs are formed in the uncoated part of the current collector by extending the current collector at predetermined intervals and, thereafter the active material layer is pressed in the thickness direction.
However, in such a manufacturing method and electrode, a part on which no active material layer is arranged is located between the tab and the active material layer, and hence there is a problem to be solved in the capacity per unit volume. Further, the mechanical strength of the tab is low, and hence a case is conceivable where the tab is deformed or strained in the manufacturing process.